Mass spectrometers are well known in the art and have particular application in sample measurements. It is also well known to provide miniaturised devices which have particular application as portable measurement systems. The use of such spectrometers is varied from the detection of biological and chemical materials, drugs, explosives and pollutants, to use as instruments for space exploration, as residual gas analysers and as instruments for process control. Mass spectrometers consist of three main subsystems: an ion source, an ion filter, and an ion counter. Since these may all be based on different principles, there is scope for a variety of systems to be constructed.
One of the most successful variants is the quadrupole mass spectrometer, which uses a quadrupole electrostatic lens as a mass filter. Conventional quadrupole lenses such as those described in Batey J. H. “Quadrupole gas analysers” Vacuum 37, 659–668 (1987) consist of four cylindrical electrodes, which are mounted accurately parallel and with their centre-to-centre spacing at a well-defined ratio to their diameter.
Ions are injected into a pupil located between the electrodes, and travel parallel to the electrodes under the influence of a time-varying hyperbolic electrostatic field. This field contains both a direct current (DC) and an alternating current (AC) component. The frequency of the AC component is fixed, and the ratio of the DC voltage to the AC voltage is also fixed. Studies of the dynamics of an ion in such a field have shown that only ions of a particular charge to mass ratio will transit the quadrupole without discharging against one of the rods. Consequently, the device acts as a mass filter. The ions that successfully exit the filter may be detected. If the DC and AC voltages are ramped together, the detected signal is a spectrum of the different masses that are present in the ion flux. The largest mass that can be detected is determined by the largest voltage that can be applied.
The resolution of a quadrupole filter is determined by two main factors: the number of cycles of alternating voltage experienced by each ion, and the accuracy with which the desired field is created. So that each ion experiences a large enough number of cycles, the ions are injected with a small axial velocity, and a radio frequency (RF) AC component is used. This frequency must clearly be increased as the length of the filter is reduced. In order to create the desired hyperbolic field, highly accurate methods of construction are employed. However, it becomes increasingly difficult to obtain the required precision as the size of the structure is reduced.
The sensitivity and hence the overall performance of a mass spectrometer is also affected by the ion flux, which is also clearly reduced as the size of the entrance pupil is decreased.
Several miniaturised quadrupole mass spectrometers have been constructed. Two examples of such instruments are based on square arrays of miniaturised electrostatic quadrupole lenses and are described in U.S. Pat. No. 5,401,962 and U.S. Pat. No. 5,719,393. The advantage of using an array is that parallel operation can recover the sensitivity lost by miniaturisation. The square array geometry is particularly efficient, because an array of N2 quadrupoles only requires (N+1)2 electrodes.
The device disclosed in U.S. Pat. No. 5,401,962 is commercialised under the brand name “The Ferran Micropole” and is available as a high-pressure residual gas analyser. It consists of a square parallel array of nine quadrupole analysers constructed using sixteen cylindrical metal rods 1 mm in diameter and 20 mm long, mounted in miniature glass-to-metal seals. The ion source is a conventional hot-cathode device. The quadrupoles are driven in parallel by a RF generator, and the ion detector consists of an array of nine Faraday collectors connected together.
The array-type quadrupole mass spectrometer described in U.S. Pat. No. 5,719,393 was developed by the Jet Propulsion Laboratory (JPL) and has electrodes that are welded to metallised ceramic jigs. The ioniser is a miniature Nier type design with an iridium-tungsten filament. The detector can be a Faraday cup or a channel-type multiplier.
Quadrupole lens arrays smaller than the devices described above have been fabricated by exposing a photoresist to synchrotron radiation and then filling the resulting mould with nickel by electroplating, in a collaboration between JPL and Brookhaven National Laboratory and described in U.S. Pat. No. 6,188,067. The lens assembly is a planar element, which is configured into a stacked structure in the complete mass spectrometer. However, there is no evidence of successful operation of the device.
A different micro-engineered quadrupole lens has been developed jointly by Imperial College and Liverpool University, and is described in U.S. Pat. No. 6,025,591. The device 100, as shown in FIG. 1, consists of four cylindrical electrodes 115 mounted in pairs on two oxidised, silicon substrates 105, that are held apart by two cylindrical spacers 120. V-shaped grooves 110 formed by anisotropic wet chemical etching are used to locate the electrodes and the spacers. The electrodes are metal-coated glass rods that are soldered to metal films 125 deposited in the grooves.
The mounting method is similar to that used to hold single-mode optical fibres in precision ribbon fibre connectors. In each case, positioning accuracy is achieved by the use of photolithography followed by etching along crystal planes to create kinematic mounts for cylindrical components. However, in the quadrupole lens, the two halves of the structure are also self-aligning. The degree of miniaturisation is only moderate, and operation has been demonstrated using devices with electrodes of 0.5 mm diameter and 30 mm length. Wirebond connectors 135 are used to provide for electrical contact to the components of the device.
Although mass filtering has been demonstrated, the method of fabrication has some disadvantages. The electrode rods require lengthy cutting, polishing and metallisation. Because the electrodes must be metal-coated everywhere, metallisation involves multiple cycles of vacuum deposition. The bonding process used to attach the electrode rods is a time consuming manual operation, requiring axial alignment. Additional fixtures are needed to hold the assembly together, and there is no axial alignment of the two substrates, which may slide over each other.
The method of fabrication also results in some important performance limitations. The oxide layer is electrically leaky, so that the drive voltage (and thus the mass range) is limited. As a result, current device performance is insufficient for applications requiring measurement of large masses (e.g. drugs or explosives detection).
There is also significant capacitance coupling to the resistive substrate, which rises as the RF frequency is increased. The device therefore forms a poor RF load, and the mass selectivity is limited. Resistance heating in the substrate also tends to melt the solder, causing the rods tend to detach from the V-grooves.
In addition, the construction forms only a mass filter, and an ion source and detector must also be added to form a complete mass spectrometer. These elements require components for creation and detection of ions, and also for accelerating and focusing ions.
There is therefore a need to provide an improved mass spectrometer device, which can be easily fabricated. There is a further need to provide an array-type device, which could be used to increase the currently low instrument sensitivity.